The present invention relates to an electromagnetic wave absorber for use in an anechoic chamber, etc., particularly to a flame-retardant electromagnetic wave absorber that can be easily assembled.
Recently, there have been increasing cases where electromagnetic waves emitted from electronic devices as noise interfere operations of the devices themselves or other devices. In such circumstances, electronic devices manufacturers are demanded to guarantee that their products neither suffer errors by external electromagnetic waves nor emit such electromagnetic waves as to affect nearby devices. Thus, the electronic devices are required to have an electromagnetic compatibility (EMC) to meet the above demand. A chamber used for EMC evaluation is such that metal plates are attached to its outer surface to shield the external electromagnetic waves, and that electromagnetic wave absorbers are attached to its inner surface to prevent the electromagnetic waves from reflecting. Such a chamber is referred to as xe2x80x9canechoic chamberxe2x80x9d. The anechoic chambers are classified as a larger anechoic chamber for large products such as automobiles and large electronic devices, and a compact anechoic chamber for relatively small electronic devices, etc.
Ferrite tile absorbers and carbon absorbers formed in a pyramid or wedge shape have been well known as the electromagnetic wave absorbers for use in the anechoic chambers. These absorbers are often used in combination. Although the carbon absorbers are generally minimized, correspondingly to electromagnetic wave-absorbing properties thereof and the size of the anechoic chamber, the absorbers for use in the larger anechoic chamber are even 1.5 to 2 m in height.
A xe2x80x9cMT-type carbon absorberxe2x80x9d and a xe2x80x9chollow-type carbon absorberxe2x80x9d have been well known. The full-type carbon absorber is generally obtained by expanding and molding a resin such as urethane resin penetrated with carbon. The hollow-type carbon absorber is generally obtained by assembling or bending carbon mixture boards, usable for the purposes of reducing weight and cost, etc. However, it is remarkably troublesome to produce the carbon absorbers of 1.5 to 2 m in height. Further, a member used for supporting the absorbers or applying them to the anechoic chamber is limited because it cannot be made of electromagnetic wave-reflecting materials such as metals. In a case where the shorter, pyramid- or wedge-shaped carbon absorber having a small base area is applied to the anechoic chamber, etc., the number thereof per a predetermined area is increased. With respect to the hollow-type carbon absorber, its assembling steps are increased with the number to unavoidably increase costs.
With regard to the immunity test recently carried out in the anechoic chamber, the electromagnetic waves with a strong electrical field are radiated to the electromagnetic wave absorber. In general, the electromagnetic wave absorber is extremely heated in the test because electromagnetic energy of the electromagnetic waves is converted to thermal energy. Therefore, the electromagnetic wave absorber is required to have excellent heat resistance and flame retardance. This is also desirable from an architectural point of view, when the electromagnetic wave absorber is applied to the inner wall of the anechoic chamber. However, because most of the conventional carbon electromagnetic wave absorbers are made of carbon dispersed in resins, they are poor in flame retardance, often burning to generate a toxic combustion gas. Though a flame-resistant material may be added to the absorber to increase the flame resistance, the material often causes aging degradation of the absorber for a long time.
To solve the problems mentioned above, carbon electromagnetic wave absorbers comprising inorganic materials such as ceramic fibers, calcium carbonate, cement mortar, etc. were developed. However, these absorbers newly cause problems such as high material cost, heavy weight, poor formability, etc.
Accordingly, an objective of the present invention is to provide a flame-retardant electromagnetic wave absorber that exhibits excellent formability and electromagnetic wave-absorbing properties, and can be easily assembled and applied.
As a result of intense research in view of the above objective, the inventors have found that an electromagnetic wave absorber, which is produced by forming a wave-absorbing body in a pyramid shape by fitting (or engaging) a plurality of wave-absorbing plates into each other and disposing a base plate on the bottom thereof, exhibits excellent electromagnetic wave-absorbing properties, formability and flame retardance.
Thus, an electromagnetic wave absorber of the present invention comprises a wave-absorbing body and a base plate supporting the bottom thereof, wherein the wave-absorbing body is formed in a pyramid shape by fitting polygonal wave-absorbing plates into each other.